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Homework Help: Force needed to push box down incline plane

  1. Sep 16, 2008 #1
    1. The problem statement, all variables and given/known data
    a) if a force of 86N parallel to the surface of a 20 degree incline plane will push a 120N block up the plane at constant speed, what force parallel to the plane will push it down at constant speed?


    2. Relevant equations
    [tex]\Sigma[/tex]F(X)=F(A)-F(fr)-mgcos[tex]\vartheta[/tex]=0
    [tex]\Sigma[/tex]F(Y)=F(n)-mgsin[tex]\vartheta[/tex]=o
    where F(A)= force applied, F(fr)= friction force, F(n)= normal force, mg=weight, F(X)= sum of forces in x direction, and F(Y)- sum of forces in y direction


    3. The attempt at a solution
    normal force=41.0424
    friction force=26.73
    my answer is -86.0331
     
    Last edited: Sep 16, 2008
  2. jcsd
  3. Sep 16, 2008 #2
    my coefficient of friction for this problem=.652
     
  4. Sep 16, 2008 #3
    could someone please check my answers?
     
  5. Sep 17, 2008 #4
    Hi
    Writing out the equation for the force to push up the block with constant speed, i.e. net force is zero.
    120 sin 20(component of weight along incline) +120 cos 20 * u(Force of friction, u is the coefficient)=86
    solve for u
    u=0.399
    Now for the second part:
    120 sin 20 +F=120 cos 20 *u
    F=120 sin 20-120 cos 20 *u
    F=3.9 N
    dos this help?????
     
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